Contamination of materials is a factor in many manufacturing processes, and is of particular concern in the fabrication of integrated circuits. In general, integrated circuit technology is based on the ability to form numerous transistor structures on a single semiconductor substrate. Typically, multiple integrated circuits will be formed on a single silicon wafer, the wafer providing the semiconductor substrate for the circuits. The intricacy of the circuits and the large number of steps involved in the fabrication make it essential that each of the process steps be tightly controlled and meet very stringent specifications to prevent any type of contamination. To increase the purity and hence the quality and reliability of manufacturing processes, most are done in sealed rooms or chambers, where the environment, including temperature, pressure and purity of liquids or gases introduced can be controlled. One of the biggest sources of contamination occurs when the product being manufactured is transferred from one area of the manufacturing process to the next, which necessitates opening a door or valve to introduce the material into the next chamber or room. When the door or valve is open, if there is a pressure differential between the chambers, the potential for contamination is increased. Processing fluid and/or gas will rush from the chamber with the higher pressure to the chamber with the lower pressure to equalize the pressure, bringing solids and liquids along with the gas, which may cause such problems as condensation and particulate contamination on the material being manufactured.
The solutions used to mitigate the above-identified problem have included: reducing the amount of time that the door or valve is opened to reduce the amount of contaminants that enter the chamber; making the transfer in two stages; utilizing an intermediate or transfer chamber into which only one of the other chambers is opened at a given time; placing calibrated pressure sensors in each chamber; and tying the pressure sensors into the valve-opening mechanisms in both chambers to obtain a desired, consistent pressure balance before the valve between the chambers is opened. However, these solutions, separately or in combination, do not always adequately resolve the aforementioned problems.
Even minimizing the time the door or valve is open will result in some contamination, especially if the pressure is not equalized between the two chambers before opening the door or valve. One of the biggest problems in equalizing the pressure is keeping the pressure sensors calibrated, as sensors tend to drift in calibration over time. If the pressure sensor in either chamber is out of calibration, the pressure between the two chambers will not actually be equal, and when the door or valve is open, the gas will rush from the higher to lower pressure chamber to equalize the pressure. The effective drift of the pressure sensors is actually doubled if the two sensors drift calibration in opposite directions. If a sensor has drifted, process overseers are generally unaware of the problem until a rush of gas between the chambers has occurred, resulting in contamination of a manufactured product. In order to re-calibrate sensors that have drifted, the manufacturing process generally has to be shut down, and the sensor taken off-line to be calibrated, resulting in production down-time.
U.S. Pat. No. 5,808,175 issued Sep. 15, 1998 to Shen-Yan Chang discloses a method of temporarily, manually mounting a second, in-line calibrated sensor to the same chamber for the purpose of monitoring or correcting the first sensor. However, Chang only utilizes the second sensor for comparison to the readings obtained from the first sensor for the same chamber, to determine if it needs replacing. If there is drift in the sensors used to read the pressure in different chambers, a situation may still occur wherein the pressure differential between two chambers is such that a rush of gas and contaminants occurs when the door or valve between the two chambers is opened.
It would, therefore, be desirable to be able to provide a method and apparatus wherein the pressure in the two chambers between which materials are being transferred can be kept equal so that there will not be a rush of gas between the two chambers when the door or valve is opened.